Microdenominated cellulose (MDC) is produced from fibrous cellulosic material that has been extensively refined and converted into a dispersed tertiary level of structure, thereby achieving certain desirable properties attendant upon such structural change.
MDC is prepared by repeatedly passing a liquid suspension of fibrous cellulose through a zone of high shear, which is defined by two opposed surfaces, with one of the surfaces rotating relative to the other, under conditions and for a length of time sufficient to render the suspension substantially stable and to impart to the suspension a Canadian Standard Freeness that shows consistent increase with repeated passage of the cellulose suspension through the zone of high shear.
The production of MDC can be efficiently carried out using standard refining equipment, e.g. a double disk refiner, operated in a way differing from the conventional use of this equipment in refining pulp for paper manufacture. Whereas paper manufacture calls for minimum damage to the fiber during refining and a Canadian Standard Freeness consistent with good drainage of water from the pulp, the same equipment is used to achieve the opposite effect in preparing MDC, i.e., a high degree of disintegration of the fiber structure, which results in a cellulose product having very high surface area and high water absorbency. The degree of disintegration is sufficiently severe that, as refining continues beyond that level normally used for paper manufacture (a Canadian Standard Freeness value approximating 100), a reversal of the Canadian Standard Freeness values occurs. The reason for this reversal is that the dispersed fiber becomes sufficiently microdenominated that gradually greater amounts of fiber begin to pass through the perforated plate of the Canadian Standard Freeness tester with water, thus leading to a progressive increase in the measured value as refining continues. Continuation of refining ultimately results in essentially all of the refined fiber readily passing through the perforated plate with water. At this stage of processing, the measured Canadian Standard Freeness value is typical of that for unimpeded passage of water through the perforated plate of the test unit.
Whereas a single stage, and at most two stages are used for conventional refiner processing in paper manufacture, the process of this invention requires multiple passages of the pulp through the zone of high shear, which may typically involve ten to forty passages.
In paper manufacture beating or refining increases the area of contact between dispersed fibers by increasing the surface area through dispersion and fibrillation. MDC manufacture applies and extends such processing to a much greater degree. It is believed that the extent of refinement needed to achieve this high degree of fibrillation leads to a concomitant disassembly of tertiary structure, and perhaps even secondary structure. The result is an ultrastructurally dispersed form of cellulose with very high surface area.
The product of the invention, MDC, is characterized by a settled volume greater than about 50% after twenty-four hours, as based on 1% by weight aqueous suspension, and water retention greater than about 500%. Procedures for determining the settled volume and water retention values of MDC are described in detail below. Details regarding the preparation of microdenominated cellulose (MDC) are set forth in our copending U.S. patent application Ser. No. 08/089,683, filed Jul. 9, 1993 entitled "Process for Making Microdenominated Cellulose" in the names of Michael K. Weibel and Richard S. Paul, which is commonly owned and filed concurrently with the present application. The entire disclosure of the aforesaid related application is incorporated in the present specification by reference as if set forth herein in full. Because the extent to which it is refined, MDC has a highly fibrillated structure resulting in a very high surface area and the ability to form stable gels. Many of the uses contemplated for MDC in food, pharmaceutical, cosmetics, and the like are best served by providing MDC as a dried product that can be readily rehydrated and redispersed, whereby it exhibits properties approximating those of never dried MDC, i.e. MDC as discharged from the refiner or prior to drying. Unfortunately, this favorable structure and the desirable properties associated therewith are lost or greatly diminished upon drying the material. This occurs as a result of a partially irreversible collapse of the structure of the cellulose fibers due to shrinkage forces exerted during drying (hornification). The favorable dispersibility, hydration and viscosity properties of MDC are lost or substantially modified depending on the severity of drying.
A number of techniques have been developed heretofore to avoid or substantially lessen the adverse effects of drying on cellulose. They include, among others, the use of additives, solvent replacement of water and modified drying techniques. The latter two approaches are described in U.S. Pat. No. 3,023,104. Water can be displaced by a water miscible organic compound of low molecular weight such as methanol, ethanol, propanol, etc. followed by evaporation of the displacement fluid. Modified drying methods include spray drying in vacuum or air up to 100 to 105 degrees Centigrade, freeze drying and drum drying.
Most additives are intended to prevent drying stress or hornification by inhibiting hydrogen bonding of the cellulose fibrils. As disclosed in U.S. Pat. No. 4,481,076, the additive forms hydrogen bonds or complexes with the cellulose fibrils and prevents them from bonding to each other during drying; thus, the cellulose fibrils remain readily accessible to water and easily rehydrated. To perform this function the additive must be capable of substantially inhibiting the hydrogen bonding between the fibrils of the cellulose. Among the compounds found as useful additives are polyhydroxy compounds including particularly carbohydrates or carbohydrate-like compounds. These additive compounds must be used in substantial quantities, generally at least one half of the dried weight of the microfibrillated cellulose and preferably at least equal to microfibrillated cellulose weight in order to achieve the desired effect.